Work apparatus having a braking arrangement

ABSTRACT

A work apparatus having a work tool and having a combustion engine which drives the work tool has a start enrichment device for the combustion engine and a braking arrangement for the work tool. The work apparatus has an operating mode selector for actuating the start enrichment device. The start enrichment device actuates the braking arrangement in at least one start position. Low actuating forces and a simple configuration are achieved when the braking arrangement is a differential band brake.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority of German patent application no. 10 2012 012 798.7, filed Jun. 28, 2012, the entire content of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

DE 33 08 400 A1 corresponds to DE 83 06 837 U1 and discloses a motor-driven saw wherein the operating mode selector is connected via a Bowden cable to a lever part which acts on the band brake of the motor-driven saw. Comparatively great operating forces are required to engage and release the band brake, with the result that the actuation of the operating mode selector is not ergonomic for the user.

U.S. Pat. No. 4,460,072 discloses a motor-driven saw which has a braking arrangement which is to be engaged via an actuating bracket. The braking arrangement comprises a lever, to which both ends of the brake band are fixed. The brake band tightens itself without the external action of force, as soon as it bears against the brake drum at one point.

U.S. Pat. No. 4,226,312 discloses a band brake for a lawnmower, which band brake can be connected, for example, to the dead man's lever of the lawnmower and is actuated if the dead man's lever is released. The brake can be configured as a differential band brake, in order to achieve a self-boosting action.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a work apparatus having high operating comfort and in which the braking arrangement is always actuated when the operating mode selector is in the start position.

The work apparatus of the invention includes: a work tool; a combustion engine configured to drive the work tool; a start enrichment unit for the combustion engine; a brake arrangement configured for the work tool; an operating mode selector configured to actuate the start enrichment unit; the start enrichment unit having a start position and being configured to actuate the brake arrangement in the start position; and, the brake arrangement being a differential band brake.

A differential band brake is a brake in which both ends of a brake band are mounted movably, to be precise at different spacings from a pivot axis. The ends of the brake band are coupled to one another in such a way that, in the case of an actuation of the brake, one end of the brake band moves in the release direction and the other end of the brake band moves by a greater distance in the direction of the actuated position. By virtue of the fact that the actuation of the two ends of the brake band is coupled to one another, the system acts in a self-boosting manner. As a result, only low actuating forces are required. The engagement and release of the brake via the operating mode selector is possible ergonomically as a result.

The differential band brake advantageously has a brake band which wraps around a brake drum at least partially. The brake band has a first end region and a second end region which do not bear against the brake drum. The differential band brake advantageously has a brake lever which is mounted such that it can be pivoted about a pivot axis. The first and the second end region of the brake band are held on the brake lever, in particular, on opposite sides of the pivot axis. This results in a simple construction. The first end region advantageously has a first spacing from the pivot axis of the brake lever, and the second end region has a second, smaller spacing from the pivot axis. Here, both spacings are measured perpendicularly with respect to the longitudinal extent of the end region. The spacings correspond to the lever arm, by way of which the end region of the brake band exerts a moment on the brake lever about the pivot axis. The rotational direction of the brake drum advantageously runs along the brake band from the first to the second end section.

The brake lever is advantageously spring-loaded in the direction of the non-actuated position of the differential band brake. The release of the differential band brake with a low exertion of force is possible as a result. Only a small actuating force is required in any case during the engagement of the differential band brake, since the differential band brake tightens automatically as soon as the brake band comes into contact with the brake drum.

The start enrichment device advantageously has two start positions. One of the start positions is advantageously a warm start position and the other is a cold start position. The differential band brake is advantageously actuated in both start positions. By virtue of the fact that a warm start position and a cold start position are provided, the combustion engine can be started in a manner which is adapted to the engine temperature, which results in improved start behavior and high operating comfort. By virtue of the fact that the differential band brake is actuated in both start positions, the work tool is not driven in both start positions and does not run together with the engine.

In order to engage the start position, for example on a carburetor of the combustion engine, it is provided that the operating mode selector has an actuating element which acts on a choke element of the combustion engine. The choke element can be, for example, a choke flap or choke roller which is arranged in a carburetor. However, a choke element can also be provided in an air filter, via which the combustion air is sucked into the combustion engine. Another design of the choke element can also be expedient.

The operating mode selector advantageously has two sections which can be moved with respect to one another. Here, a first section advantageously acts on the differential band brake and a second section has the actuating element. The differential band brake is to have the same actuated position in both start positions. However, the choke element is usually closed to a greater extent in the cold start position than in the warm start position, with the result that two different positions are required for the choke element. This can be achieved in a simple way by the two sections of the operating mode selector, which sections can be moved with respect to one another. When the operating mode selector is moved from the operating position into the warm start position, the two sections of the operating mode selector are advantageously moved together with it, whereas, during the adjustment of the operating mode selector from the warm start position into the cold start position, the second section of the operating mode selector is moved with respect to the first section.

A handle section for the operator is advantageously formed on the second section of the operating mode selector. During the adjustment of the operating mode selector from the warm start position into the cold start position, it is provided that the second section moves with respect to the first section and the first section of the operating mode selector does not change its position.

A simple design results if the operating mode selector is configured as a lever which is mounted such that it can be pivoted about a pivot axis. The combustion engine advantageously drives the work tool via a centrifugal clutch. A separate brake drum can be avoided if the clutch drum of the centrifugal clutch is utilized as brake drum. A simple construction is achieved as a result.

The work apparatus is advantageously a motor-driven saw and the work tool is advantageously a saw chain. The motor-driven saw advantageously has a hand protection bracket for actuating a braking arrangement for the saw chain. The braking arrangement for the saw chain is advantageously the differential band brake, and the hand protection bracket acts on the differential band brake. As a result, only one braking arrangement is required, on which both the hand protection bracket and the operating mode selector act. However, it can also be advantageous that the motor-driven saw has a second braking arrangement for the saw chain, and that the hand protection bracket acts on the second braking arrangement. By virtue of the fact that the two braking arrangements are configured independently of one another, the braking arrangements can be arranged independently of one another. By virtue of the fact that the differential band brake is not the service brake of the work apparatus, in particular of the motor-driven saw, the differential band brake can be of self-locking design. Very low operating forces for the differential band brake can be achieved as a result.

On account of the self-locking action, a standstill of the work tool can already be achieved with a very low actuating force. Since the differential band brake is engaged before the combustion engine is started, that is to say when the work tool is at a standstill, the differential band brake is subjected to scarcely any or no wear. The differential band brake can therefore be of simple construction, can have a low weight and can be produced inexpensively. Since the differential band brake is not a service brake, and is therefore not active during operation, it does not experience any excessive increase in moment during operation. Here, the excessive increase in moment denotes the self-boosting action of the differential band brake.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the drawings wherein:

FIG. 1 shows a perspective view of a brushcutter;

FIG. 2 shows a schematic section view through the housing of the brushcutter of FIG. 1;

FIG. 3 shows a perspective, partially sectioned view of the brushcutter in an operating position;

FIG. 4 shows the brushcutter of FIG. 3 in a partially sectioned side view;

FIG. 5 shows an enlarged side view of the brake lever;

FIG. 6 shows a schematic view of the carburetor of the brushcutter in the operating position;

FIG. 7 shows a perspective, partially sectioned view of the brushcutter in a warm start position;

FIG. 8 shows the brushcutter of FIG. 7 in a partially sectioned side view;

FIG. 9 shows a schematic view of the carburetor of the brushcutter in a warm start position;

FIG. 10 shows a perspective, partially sectioned view of the brushcutter in a cold start position;

FIG. 11 shows the brush cutter of FIG. 10 in a partially sectioned side view;

FIG. 12 shows a schematic view of the carburetor in a cold start position;

FIG. 13 shows a schematic side view of a motor-driven chain saw;

FIG. 14 shows a schematic sectional view of the motor-driven saw of FIG. 13; and,

FIG. 15 shows a schematic sectional view of one exemplary embodiment of a motor-driven chain saw.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows a handheld brushcutter 1 as exemplary embodiment for a work apparatus. The brushcutter 1 has a motor housing 2 which is connected via a guide wand 3 to a cutter head 4. A work tool 5, a blade in the exemplary embodiment, is mounted on the cutter head 4 such that it can be rotated about a rotational axis 6. Instead of a blade, another cutting tool, for example a cutting filament, can also be provided. The work tool 5 is covered partially by a guard 7. Two handles 8 are fixed on the guide wand 3 on a handlebar-like grip. The brushcutter 1 can be guided via the two handles 8.

FIG. 2 schematically shows the construction of the drive of the brushcutter 1. In order to drive the work tool 5 rotationally about the rotational axis 6, the brushcutter 1 has a combustion engine 9 which is arranged in the motor housing 2. The combustion engine 9 can be started via the starter device 13 which can be configured as a pull-rope starting device to be actuated manually or as an electrically operating starter device. The combustion engine 9 has a spark plug 10 which ignites the fuel/air mixture, and a piston 11 which drives a crankshaft 12 of the combustion engine 9 rotationally about a rotational axis 50. The starter device 13 acts on the crankshaft 12. A fan wheel 14 is fixed on the crankshaft 12 on that side of the combustion engine 9 which lies so as to face away from the starter device 13. The fan wheel 14 advantageously carries magnets (not shown) which induce the ignition voltage for the spark plug 10 in an ignition module 23. Moreover, a centrifugal clutch 15 is fixed on the crankshaft 12. On the output side, the centrifugal clutch 15 has a clutch drum 16 which also serves as brake drum and on the outer side of which a brake band 17 runs. The clutch drum 16 is connected fixedly via a coupling element 18 to the drive shaft 19 so that the latter rotates therewith. The drive shaft 19 passes through the guide wand 3 and drives the work tool 5, advantageously via an angular gear mechanism which is arranged in the cutter head 4.

As FIG. 3 shows, the brushcutter 1 has an operating mode selector 22. FIG. 3 shows the operating mode selector 22 in the operating position 52. The operating mode selector 22 has a handle section 51 which protrudes out of the housing 2 and which can be gripped by the operator. The operating mode selector 22 acts via an actuating cam 35 on a choke lever 34 and sets start positions of a carburetor 21 as a result. The combustion engine 9 draws in combustion air and fuel via the carburetor 21.

Together with the actuating cam 35 and the choke lever 34, the operating mode selector 22 forms a start enrichment device 57 of the combustion engine 9. The operating mode selector 22 acts, moreover, on a differential band brake 56, which includes a brake lever 24 mounted so as to be pivotable about a pivot axis 25, the brake band 17 and the clutch drum 16. A spring 26, which is configured in the exemplary embodiment as a helical tension spring, acts on the operating mode selector 22 and the brake lever 24. The spring 26 loads the operating mode selector 22 and the brake lever 24 in the direction of the operating position 52 of the start enrichment device 57 and in the direction of the non-actuated position of the differential band brake 56. In the non-actuated position of the differential band brake 56, the clutch drum 16 can be rotated freely, with the result that the work tool 5 can be driven. In its actuated position, the differential band brake 56 blocks the clutch drum 16, with the result that the work tool 5 is at a standstill. As FIG. 3 also shows, a fuel tank 20 is integrated into the housing 2, from which fuel tank 20 the combustion engine 9 is supplied with fuel. The fuel tank 20 can also be of separate configuration and can be arranged on or in the motor housing 2.

As FIG. 4 shows, the brake band 17 has a first end region 27 which is held at an attachment location 29 on the brake lever 24. Here, the attachment location 29 is that location of the brake band 17 which adjoins the end region 27 which is arranged freely between the clutch drum 17 and the brake lever 24. That region of the brake band 17 which adjoins the attachment location 29 is wrapped around an attachment lug 47 of the brake lever 24, as FIG. 5 also shows. Here, the brake band 17 can be arranged in a recess 59 of the brake lever 24. The brake band 17 can be fixed on the attachment lug 47 or can be rotatable about the attachment lug 47. The brake band 17 has a second end region 28 which is fixed at an attachment location 30 on the opposite side of the pivot axis 25. That section of the brake band 17 which adjoins the attachment location 30 is likewise wrapped around an attachment lug 47. This end of the brake band 17 can also be held in a recess 59 of the brake lever 24.

The end regions 27 and 28 are the regions which run approximately tangentially with respect to the outer circumference of the clutch drum 16. The attachment locations 29 and 30 are the locations of the brake band 17 which adjoin the end regions 27 and 28 and at which the brake band 17 is fixed. The brake band 17 is likewise fixed in the region which adjoins the attachment locations 29 and 30, but the course of the brake band 17 in this region can be selected in a suitable manner and has no appreciable influence on the braking forces on the clutch drum 16. The brake band 17 can also be held rotatably on the attachment lug 47. The attachment locations 29 and 30 are then those locations of the brake band 17 which bear against the attachment lug 47 and adjoin the end regions 27 and 28.

During operation, the clutch drum 16 rotates about the rotational axis 50 in a rotational direction 31 which is shown in FIG. 4. The rotational direction 31 runs from the first end region 27 on the outer circumference of the clutch drum 16 along the brake band 17 as far as the second end region 28. Accordingly, in the state in which it is set against the brake drum 16, the brake band 17 is driven by the first end region 27 in the direction of the second end region 28, until the brake band 17 is wrapped firmly around the clutch drum 16.

As FIG. 4 shows, the operating mode selector 22 is configured as a lever which is mounted such that it can be pivoted about a pivot axis 32. On that side of the pivot axis 32 which faces away from the handle section 51, the operating mode selector 22 has a connecting pin 33, at which it is connected to the brake lever 24 in an articulated manner. The connecting pin 33 is arranged so as to lie opposite the second end region 28 in relation to the pivot axis 25 of the brake lever 24. If the connecting pin 33 moves away from the clutch drum 16, the second end region 28 likewise moves away from the clutch drum 16, that is, to the top in FIG. 4, and the first end region 27 moves downward toward the clutch drum 16.

The arrangement of the end regions 27 and 28 is shown on an enlarged scale in FIG. 5. FIG. 5 shows the brake lever 24 from the side which lies adjacently with respect to the combustion engine 9. The extension of the first end region 27 has a spacing (a) from the pivot axis 25. The spacing (a) is measured perpendicularly with respect to the extension of the first end region 27. The second end region 28 has a spacing (b) from the pivot axis 25, which spacing (b) is measured perpendicularly with respect to the end region 28 and is considerably greater than the spacing (a). The spacing (b) is a multiple of the spacing (a). The spacing (b) is advantageously approximately from 2 times to approximately 20 times the spacing (a). Viewed at the elevation of the pivot axis 25, the extensions of the end regions 27 and 28 run in the longitudinal direction of the brake lever 24 on different sides of the pivot axis 25. As a result, with the same force on the brake band 17, the end regions 27 and 28 exert torques in opposed directions on the brake lever 24.

FIG. 6 schematically shows the carburetor of the combustion engine 9. An intake channel 36 is formed in the carburetor 21, through which intake channel 36 combustion air is drawn into the combustion engine 9 in a flow direction 37. At least one fuel opening (not shown) advantageously opens into the intake channel 36 in the carburetor 21, via which fuel opening fuel is fed to the combustion air. A throttle element, a throttle flap 38 in the exemplary embodiment, is arranged in the carburetor 21. The throttle flap 38 is arranged pivotably with a throttle shaft 39 and can be actuated by the operator via a throttle lever (not shown). A choke element, namely a choke flap 40, is arranged upstream of the throttle flap 38 in relation to the flow direction 37. The choke flap 40 is mounted pivotably with a choke shaft 41. The choke lever 34, which is arranged on the outer side of the housing of the carburetor 21 as FIG. 4 shows, is connected fixedly to the choke shaft 41 so as to rotate therewith. The actuating cam 35 acts on the choke lever 34. In the operating position 52 which is shown in FIG. 6, the choke flap 40 is not pivoted and is oriented approximately parallel to a longitudinal center axis 42 of the intake channel 36.

FIGS. 7 and 8 show the start enrichment device 57 in a warm start position 53. In order to set the warm start position 53, the operator has to pivot the operating mode selector 22 on the handle section 51 in the direction of the arrow 58 which is shown in FIG. 4. As FIG. 8 shows, the operating mode selector 22 is actuated in the direction of the arrow 58 counter to the force of the spring 26. During the pivoting of the operating mode selector 22, the actuating cam 35 pivots the choke lever 34. At the same time, the connecting pin 33 drives the brake lever 24 and pivots the latter about the pivot axis 25. Here, the brake band 17 bears against the outer circumference of the clutch drum 16. The differential band brake 56 is self-boosting on account of the arrangement of the end regions 27 and 28. As soon as the brake band 17 comes into frictional contact with the clutch drum 16, the clutch drum 16 drives the brake band 17. As a result, the attachment location 29 (FIG. 4) is drawn toward the clutch drum 16. As a result, the brake lever 24 is pivoted further into the actuated position at the same time. Since the attachment location 29 covers a smaller path during the pivoting of the brake lever 24 than the attachment location 30, the differential band brake 56 continues to tighten.

The different paths result from the different spacings (c, d) of the attachment locations 29 and 30 from the pivot axis 25. As FIG. 8 shows, the spacing (c) of the attachment location 29 from the pivot axis 25 is considerably smaller than the spacing (d) of the attachment location 30 from the pivot axis 25. Here, the spacings (c) and (d) are measured perpendicularly with respect to the pivot axis 25.

The brake lever 24 is mounted on a retainer 48 which is shown in FIGS. 7 and 8. The retainer 48 is fixed to a fastening screw 49 of the ignition module 23 on the combustion engine 9, to be precise on the cylinder of the combustion engine 9. The retainer 48 advantageously engages over the brake lever 24 and is fixed to the housing 2 of the brushcutter 1 at least at one further location which is situated in front of the sectional plane in FIG. 8 and is not shown. This results in high stability of the arrangement, with the result that the braking forces which act on the brake lever 24 can be absorbed satisfactorily. The retainer 48 is advantageously additionally fixed to the housing 2 in the region of the pivot axis 32 of the operating mode selector 22. Further or other attachment locations for the retainer 48 can also be advantageous. On its side which lies so as to face away from the combustion engine 9, the clutch drum 16 is advantageously mounted in the motor housing 2 via a bearing 75 (shown diagrammatically in FIG. 7) which is configured, in particular, as a needle bearing. In addition, the clutch drum 16 is mounted in the usual way on the crankshaft 12, as FIG. 2 shows schematically. Wobbling of the clutch drum 16 during operation can be avoided by way of the additional mounting in the motor housing 2.

As FIG. 9 shows, in the warm start position 53, the actuating cam 35 has pivoted the choke lever 34 with respect to the operating position 52. As a result, the choke flap 40 has also been pivoted with respect to the longitudinal center axis 42. The choke flap 40 considerably reduces the free flow cross section of the intake channel 36 in the warm start position 53. As a result, the supplied quantity of combustion air is reduced and therefore also the fuel/air mixture which is drawn into the combustion engine 9 is enriched.

As FIGS. 7 and 8 show, the operating mode selector 22 has a first section 44 which carries the connecting pin 33, and a second section 45. As FIG. 8 shows, the connecting pin 33 is guided in a slot 43 of the brake lever 24. As a result, relative movements of the operating mode selector 22 and the brake lever 24 in the direction of the connecting line of the pivot axes 25 and 32 are compensated for. The second section 45 of the operating mode selector 22 is mounted on the first section 44 such that it can be moved in the longitudinal direction of the operating mode selector 22. In order to adjust the start enrichment device 57 from the warm start position 53 which is shown in FIGS. 7 and 8 into the cold start position 54 which is shown in FIGS. 10 and 11, the operator has to pull the second section 45 to the outside in the direction of the arrow 46 which is shown in FIG. 8. As a result, the position of the first section 44 is not changed, with the result that the first section 44 and therefore also the brake lever 24 are in the same position in the warm start position 53 and in the cold start position 54.

The differential band brake 56 is actuated both in the warm start position 53 and in the cold start position 54. During the adjustment of the operating mode selector 22 between the warm start position 53 and the cold start position 54, the differential band brake 56 is not released, but rather remains actuated. As FIGS. 10 and 11 show, the actuating cam 35 is moved further by the movement of the second section 45 of the operating mode selector 22 and pivots the choke lever 34 further. As FIG. 12 shows, the choke flap 40 in the cold start position 54 is arranged approximately in its closed position, with the result that the flow cross section of the intake channel 36 is reduced very considerably and considerably more pronounced enrichment of the fuel/air mixture which is fed to the combustion engine 9 results than in the warm start position 53.

As FIG. 11 shows, the second section 45 is mounted in a longitudinally displaceable manner on a guide 55 of the first section 44. This results in a simple construction. The movement of the second section 45 with respect to the first section 44 is advantageously blocked in the operating position 52, for example by a corresponding design of the housing 2, with the result that the operator can first of all set the warm start position 53 and only subsequently the cold start position 54.

In the warm start position 53 and the cold start position 54, the throttle flap 38 is advanced slightly in comparison with the non-actuated position, with the result that the throttle flap 38 reduces the free flow cross section of the intake channel 36 to a less pronounced extent than in the operating position 52 which is shown in FIG. 6. The throttle flap 38 can be pivoted as a function of the position of the choke flap 40 via corresponding levers which are advantageously arranged on that side of the carburetor 21 which lies at the rear and is not shown in the figures.

FIG. 13 shows a motor-driven chain saw 60 as an exemplary embodiment for a hand-held work apparatus. The motor-driven chain saw 60 has a housing 61, on which a rear handle 62 is arranged. A throttle lever 63 and a throttle lever lock 64 are mounted pivotably on the rear handle 62. The operating mode selector 22 protrudes out of the housing 61 adjacently with respect to the rear handle 62. Here, identical designations as in the preceding figures denote elements which correspond to one another. The motor-driven chain saw 60 has a guide rail 67 which protrudes to the front on that front end 74 of the housing 61 which faces away from the rear handle 62. A saw chain 68 is arranged so as to circulate on the guide rail 67, which saw chain 68 is driven by the combustion engine 9 which is arranged in the housing 61. A bale handle 65 is arranged adjacently with respect to the front end 74 of the housing 61. A hand protection bracket 66 which is mounted pivotably is arranged on that side of the bale handle 65 which faces the saw chain 68. The combustion engine 9 which is arranged in the housing 61 has a spark plug 10 and an ignition module 23. In order to suck in fuel/air mixture, the combustion engine 9 has a carburetor 21, on which the operating mode selector 22 acts in order to set a warm start position 53 and a cold start position 54.

FIG. 14 diagrammatically shows a section through the housing 2 of the motor-driven chain saw 60. The combustion engine 9 has a piston 11 which drives a crankshaft 12 rotationally about a rotational axis 50. A fan wheel 14 is arranged on the crankshaft 12, on the outer circumference of which fan wheel 14 the ignition module 23 is arranged. A starter device 13 for the combustion engine 9 is arranged on that side of the fan wheel 14 which faces away from the combustion engine 9. A centrifugal clutch 15 is arranged on the opposite side of the combustion engine 9. The clutch drum 16 of the centrifugal clutch 15 is wrapped around at least partially by a brake band 17. A differential band brake 56 acts on the brake band 17, which differential band brake 56 is shown diagrammatically in FIG. 14 and can be configured, for example, in accordance with the differential band brake 56 of the preceding exemplary embodiment. The hand protection bracket 66 acts on the brake lever 24 of the differential band brake 56 via an actuating element 69 (not shown in greater detail) which can be formed, for example, by one or more levers or a toggle lever arrangement. The differential band brake 56 can be actuated and, as a result, the clutch drum 16 can be blocked both by pivoting of the hand protection bracket 66 and by adjustment of the operating mode selector 22 from the operating position 52 into the warm start position 53. A drive pinion 70 for driving the saw chain 68 is fixed on the clutch drum 16. The saw chain 68 is braked when the differential band brake 56 is actuated.

FIG. 15 shows a further exemplary embodiment of the motor-driven chain saw 60. The hand protection bracket 66 acts here via an actuating element 69 on a band brake 71 which wraps around the clutch drum 16. The band brake 71 has a brake band 72 which is connected to the actuating element 69. In addition to the band brake 71, the motor-driven chain saw 60 which is shown in FIG. 15 has a differential band brake 56 which acts on a brake band 17. The brake band 17 wraps around a brake drum 73 which is arranged on the output side of the clutch drum 16. As an alternative, the clutch drum 16 can also be of extended configuration and can be wrapped around both by the brake band 17 and by the brake band 72. The brake band 17 can also wrap around the clutch drum 16 and the second brake band 72 can wrap around the brake drum 73. In the design variant which is shown in FIG. 15, the two braking arrangements, that is, the band brake 71 and the differential band brake 56, can be actuated separately of one another. As soon as at least one of the two braking arrangements is actuated, the saw chain 68 is not driven. The operating mode selector 22 also acts on the differential band brake 56 which is shown in FIG. 15.

It is understood that the foregoing description is that of the preferred embodiments of the invention and that various changes and modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims. 

What is claimed is:
 1. A work apparatus comprising: a work tool; a combustion engine configured to drive said work tool; said combustion engine having a choke element; a start enrichment unit for said combustion engine; said start enrichment unit being operatively connected to said choke element so as to cause said choke element to assume a start position; an operating mode selector configured to actuate said start enrichment unit; a brake arrangement configured for said work tool; said brake arrangement being a differential band brake and including a brake band; said brake band having first and second movably mounted ends; said start enrichment unit having an operating position, a cold start position and a warm start position; said operating mode selector further having a first section and a second section configured to be movable with respect to one another; said first section being configured to act on said differential band brake in both said cold start position and said warm start position; said second section including an actuating element configured to act on said choke element in both said cold start position and said warm start position; said operating mode selector being configured such that said first section and said second section move conjointly when said operating mode selector is adjusted from said operating position to said warm start position; and, said operating mode selector further being configured such that said second section moves relative to said first section when said operating mode selector is adjusted from said warm start position to said cold start position while said first section remains stationary.
 2. The work apparatus of claim 1, wherein: said differential band brake has a brake drum and said brake band has a first end region extending from said first end and a second end region extending from said second end; and, said brake band is configured to at least partially wrap around said brake drum in such a manner that said first end region and said second end region are in non-contact with said brake drum.
 3. The work apparatus of claim 2, wherein: said differential band brake has a brake lever defining a pivot axis and configured to be pivotable about said pivot axis; and, said first end region and said second end region of said brake band are held on opposite sides of said pivot axis.
 4. The work apparatus of claim 3, wherein: said first end region defines a first imaginary longitudinal extension and a first distance (a) measured perpendicularly from said first imaginary longitudinal extension to said pivot axis of said brake lever; said second end region defines a second imaginary longitudinal extension and a second distance (b) measured perpendicularly from said second imaginary longitudinal extension to said pivot axis of said brake lever; said distance (b) is greater than said distance (a); and, said brake drum defines a rotational direction which runs along said brake band from said first end region to said second end region.
 5. The work apparatus of claim 3, wherein: said differential band brake has a non-actuated position; and, said brake lever is spring biased in the direction of said non-actuated position of said differential band brake.
 6. The work apparatus of claim 1, wherein said operating mode selector defines a pivot axis and is configured as a lever pivotable about said pivot axis.
 7. The work apparatus of claim 2 further comprising a centrifugal clutch, wherein: said combustion engine is configured to drive said work tool via said centrifugal clutch; said centrifugal clutch has a clutch drum; and, said brake drum is said clutch drum.
 8. The work apparatus of claim 1, wherein the work apparatus is a chain saw and said work tool is a saw chain, said brake arrangement is configured for said saw chain, the work apparatus further comprising: a hand protection bracket configured to actuate said brake arrangement.
 9. The work apparatus of claim 8, wherein said hand protection bracket is configured to act on said differential band brake.
 10. The work apparatus of claim 8, wherein said brake arrangement is a first brake arrangement, the work apparatus further comprising: a second brake arrangement for said saw chain; and, a hand protection bracket configured to actuate said second brake arrangement. 